The early events in the virus infectious cycle may serve as critical targets for therapeutic intervention. These events are difficult to study due to the transient and dynamic nature of the entry process, the low signal intensity, and the lack of a systemic approach to identify the cellular components required for individual steps. Despite such difficulties, receptors for many important viral pathogens have been identified and general themes in viral attachment and entry are beginning to emerge. For instance, the envelope proteins of orthomyxoviruses, paramyxoviruses, and retroviruses are translated as single polypeptides that are subsequently processed by a basic endopeptidase to generate two polypeptides, one for receptor binding and the other for membrane fusion (13,22). The fusion activity is not triggered until following receptor-ligand interaction and may occur at the site of virus binding (retroviruses) or in the acidic environment of the endosome following virus endocytosis (influenza virus). Furin and PC7 are the known endopeptidases capable of processing gp160 of human immunodeficiency virus (HIV) into the N-terminal gp120 (surface protein) and C-terminal gp41 (transmembrane protein) in the secretory pathway (9). The gp120 remains associated with virus particles via its noncovalent interaction with gp41. Binding of gp120 to CD4, the docking receptor, is accompanied by gp41 interaction with CXCR4 or CCR5, the viral coreceptor, thus leading to fusion of the viral membrane with the plasma membrane of the target cell (1, 6).Such a cleavage event has not been documented for hepatitis B virus (HBV) or its related animal viruses, a group of enveloped hepatotropic DNA viruses (Hepadnaviridae) that, similar to retroviruses, requires reverse transcription for genome replication (7). Instead, HBV expresses three coterminal envelope proteins via alternative translational initiation. Thus, the abundant small envelope protein contains the S domain only, while the large and middle envelope proteins contain additional pre-S1/pre-S2 domains and pre-S2 domain, respectively. The cellular receptor for HBV remains enigmatic following decades of extensive search, as no binding protein for the pre-S1 domain, the candidate site for receptor engagement, turned out to confer HBV susceptibility in reconstitution experiments. Due to the difficulty in obtaining primary human hepatocytes, which may have variable susceptibility to HBV infection, it is not clear whether antibodies against some of the pre-S1 binding partners could block HBV infection. In this regard, duck hepatitis B virus (DHBV) is an ideal model to study the early events in the hepadnaviral life cycle. DHBV can infect ducklings in vivo and primary duck hepatocytes (PDH) in vitro, which remain susceptible to DHBV infection for 2 weeks if maintained in serum-free medium supplemented with dimethyl sulfoxide (DMSO) (25). DHBV expresses two envelope proteins, with a 161-amino-acid pre-S domain unique to the large envelope protein. At least two cellular proteins have been...